A visual inspection of the spark plugs will display symptoms and conditions of the engine’s performance.
The experienced technician can analyze these symptoms to track down the root cause of many problems, or to determine air/fuel ratios.
You already know that spark plugs wear out. Well, “burn up” is more like it, because when a spark jumps the gap between two electrodes, it actually burns off (erodes) minute amounts of metal from each one.
Over time, the gap grows to the point where the spark can no longer make the jump. That’s when you get misfires, poor gas mileage, lousy acceleration and, ultimately, the dreaded “Check Engine” light.